1. Field of the Invention
The present invention relates to an oxygen sensor element making use of a solid electrolyte, suitable for use in oxygen meter or exhaust gas cleaning system for vehicles.
2. Description of the Prior Art
Oxygen sensor of oxygen concentration cell type, making use of a zirconia solid electrolyte, is known as an oxygen sensor for measuring the oxygen concentration in a gas. In this type of oxygen sensor, however, it is difficult to measure the oxygen concentration in the object gas because of an extremely small electromotive force, when the difference of oxygen concentration between the reference gas and the object gas is small. In addition, the construction of the apparatus becomes complicated because of necessity for the reference gas.
Contrary to the principle of the above oxygen concentration cell, it is known that, when a voltage is applied between electrodes formed on both surfaces of the solid electrolyte, oxygen permeates through the solid electrolyte from one side (cathode side) to the other side (anode side). Therefore, if a part of the surface of one of the electrodes is closed, the rate of permeation of oxygen is decreased correspondingly in response to the closing degree even though the voltage applied thereto is maintained constant, so that the amount of the electric current between two electrodes is decreased. To the contrary, if the area of closing of one electrode surface is maintained constant, the electric current is changed in accordance with the change in the oxygen concentration. A method has been developed for measuring the oxygen concentration through detecting the change in the electric current. An oxygen sensor called "limit current type oxygen sensor" has been known already as an oxygen sensor relying upon the above-explained theory.
The oxygen sensor element of the kind described has an oxygen ion-permeable sintered body having a tabular, disk-like, cylindrical or columnar shape and electrodes formed on both sides of the sintered body and adapted to receive voltage through leads connected thereto. Porous coating layers are formed on the electrode surfaces by flame spray of a spinel type material such as MgO.Al.sub.2 O.sub.3 to restrict the permeation of oxygen. Since the oxygen permeability of the coating layer largely depends on the diameter of the porosity of the coating layer, and density of the the coating layer, as well as the thickness of the coating layer, the unevenness of thickness of the coating layer formed by the flame spray adversely affects the oxygen permeability to unstabilize electric characteristics of the element. In addition, durability is not so high because the coating layers formed by flame spray tend to peel off during long use.
Furthermore, the step of formation of the electrode is quite complicated and laborious, as will be understood from the following description. Namely, FIG. 4 shows the step of formation of the electrodes, as well as the coating layers. In this step, a sintered body of solid electrolyte is activated and chemical plating is effected on the sintered body. Then, using the chemical plating layers as electrodes, an electroplating is effected to form complete electrodes. Finally, the spinel type material is flame-sprayed onto the electrode.
The restriction of permeation of oxygen may be made by another method called "diffusion hole method" in which a casing having apertures of a predetermined size is placed on the element. This method, however, is quite complicated and impractical.